Photosensitive compositions useful in preparing flexographic printing plates are well known in the art. These compositions generally comprise (1) an addition polymerizable, non-gaseous, ethylenically unsaturated monomer or oligomer, (2) a photoinitiator or photoinitiating system activated by actinic radiation and (3) a thermoplastic elastomeric polymeric binder comprising polymerized conjugated diene monomers. Flexographic printing elements can be made from these photosensitive compositions by solvent casting, by extruding and calendering, or by pressing the compositions at an elevated temperature into the form of a layer or self-supporting sheet on a suitable casting wheel, belt or platen. The photosensitive sheet can be permanently affixed to a suitable permanent substrate. A removable coversheet is usually placed on top to protect the surface of the photosensitive sheet from contamination by dirt and dust particles during storage prior to use.
Photopolymerizable elements and processes for their use in preparing flexographic printing plates are well known in the art. See, for example, U.S. Pat. Nos. 4,323,636; 4,460,675; 4,970,037; and many others.
Flexographic printing plates can be made from the photosensitive elements described above, by exposing to actinic radiation selected portions of the photosensitive layer, usually through an image-bearing transparency. Upon exposure to actinic radiation, the ethylenically unsaturated monomer or oligomer is polymerized or crosslinked in those areas of the photosensitive layer exposed to actinic radiation, resulting in reduced solubility or swellability in developer solvents. No significant polymerization or crosslinking occurs in the unexposed areas of the photosensitive layer. Consequently, upon application of suitable developer solvent, the exposed areas remain, and the unexposed areas are removed.
The developer solvents most commonly used are organic solvents including aromatic or aliphatic hydrocarbon or halohydrocarbon solvents. Recently, it has become less desirable to use such organic solvents due to toxicity and environmental concerns. Thus, photosensitive compositions which are aqueous developable, i.e., can be washed out with aqueous solutions or plain water, have been proposed for use in preparing flexographic printing plates. Some examples include U.S. Pat. Nos. 4,177,074; 4,361,640; and 4,960,073.
Developability in aqueous alkali solutions can be achieved by using acid-containing copolymers. Developability in plain water can be achieved by using acid-containing copolymers that have been neutralized by the addition of organic or inorganic bases. For example, Mirle et al., U.S. Pat. No. 5,143,819, disclose a method of making an aqueous developable photocurable composition by reacting an acid-modified polybutadiene copolymer with a nitrogen-containing compound having a polymerizable ethylenically unsaturated group. The composition is then used to make an aqueous developable flexographic printing plate.
Aqueous developability can also be achieved using a photosensitive composition in which the binder is a core shell microgel as described by Fryd et al. in U.S. Pat. No. 5,075,192. The core shell microgel binder described by Fryd et al. is prepared in a two-step process whereby an elastomeric core is first formed and then grafted to an acid-modified copolymer. However, such microgel binders sometimes have excessive crosslinking of the elastomeric core which can degrade the elastomeric properties and/or cause processing difficulties.
Accordingly, there is a need for an improved aqueous developable photosensitive element which can be used to prepare a flexographic printing plate.